OpenStack tenantName should be able to be provided when authenticating

Details

Description

Providing a tenantName parameter when authenticating will return
a token scoped to that tenant. Some OpenStack providers will return
a token scoped to the default tenant if it is omitted. Others require
the tenantName parameter on authentication in order to return useful
endpoints. This change adds the ex_tenant_name parameter to the
OpenStack NodeDriver and passes it to the identity service if it's
present.